Device and method for determining the concentration of a substrate

ABSTRACT

An apparatus for determining the concentration of a substrate in a sample solution using an electrode system comprising a working electrode, a counter electrode, and a reaction layer which contains at least an oxidoreductase and an electron mediator and is formed on the electrode system to electrochemically measure a reduced amount of the electron mediator resulting from enzyme reaction in the reaction layer, wherein a third electrode is formed as an interfering substance detecting electrode. A current flowing between the counter electrode and the third electrode is measured which is taken as a positive error. Subsequently, voltage application between the counter electrode and the third electrode is released and a voltage for oxidizing the reduced form electron mediator is applied between the working electrode and the counter electrode to measure a current flowing between the two electrodes.

This application is a Division of No. 09/424,715 filed on Nov. 29, 1999now U.S. Pat. No. 6,340,428 claims priority under 35 U.S.C. 371 fromPCT/JP99/01706, filed on Mar. 31, 1999.

TECHNICAL FIELD

The present invention relates to a method for performing rapid and highaccuracy determination of a substrate in a sample in a simplifiedmanner.

BACKGROUND ART

As the method for quantitative analysis of sugars such as sucrose andglucose, polarimetry, colorimetry, reductiometry, and methods using avariety of chromatography have been developed. However, all of thosemethods have poor accuracy because of poor specificity to sugars. Ofthose methods, polarimetry is simple in manipulation but is largelyaffected by the temperature during operation. Therefore, polarimetry isnot a suitable method for ordinary people to carry out determination ofsugars at home or elsewhere in a simplified manner.

Apropos, various types of biosensor have been developed recently whichuse specific catalytic actions of enzymes.

In the following, a method of glucose determination will be described asone example of the method of substrate determination in a samplesolution. A generally known electrochemical method of glucosedetermination is a method which uses glucose oxidase (EC1.1.3.4;hereinafter abbreviated to GOD) and an oxygen electrode or a hydrogenperoxide electrode (for instance, “Biosensor” ed. by Shuichi Suzuki,Kodansha).

GOD selectively oxidizes a substrate β-D-glucose to D-glucono-δ-lactoneusing oxygen as an electron mediator. Oxygen is reduced to hydrogenperoxide in the presence of oxygen in the course of oxidation reactionby GOD. A decreased amount of oxygen is measured by the oxygen electrodeor, otherwise, an increased amount of hydrogen peroxide is measured bythe hydrogen peroxide electrode. The decreased amount of oxygen orincreased amount of hydrogen peroxide is proportional to the glucosecontent in the sample solution, so that glucose can be determined basedon the decreased amount of oxygen or increased amount of hydrogenperoxide.

As can be speculated from the reaction process, this method has adrawback that the measurement result is largely affected by the oxygenconcentration in the sample solution. Furthermore, measurement isimpossible in the absence of oxygen in the sample solution.

Therefore, a novel type glucose sensor has been developed which does notuse oxygen as the electron mediator but uses an organic compound or ametal complex including potassium ferricyanide, ferrocene derivatives,quinone derivatives, etc. as the electron mediator. This type of sensoroxidizes a reduced form electron mediator resulting from enzyme reactionon the electrode and determines glucose concentration contained in thesample solution based on the quantity of oxidation current. The use ofsuch organic compound or metal complex as the electron mediator in placeof oxygen enables formation of a reaction layer while exactly carrying aknown amount of GOD and either of such electron mediators in astabilized state. In this case, since the reaction layer can beintegrated in an almost dry state with the electrode system, adisposable type glucose sensor based on this technology has been drawingmuch attention currently.

The disposable type glucose sensor facilitates measurement of glucoseconcentrations with a measurement device by simple introduction of asample solution into the sensor detachably connected to the measurementdevice. Application of such technic is not limited only to glucosedetermination and can be extended to determination of other substratecontained in the sample solution.

Measurement using the sensor as described before can determine thesubstrate concentration based on a flowing oxidation current valueresulting from oxidation of a reduced form electron mediator on aworking electrode. However, when blood, a fruit juice or something likethat is used as a sample, any easy-to-oxidize substance contained in thesample solution, such as ascorbic acid, uric acid, etc. is concurrentlyoxidized on the working electrode together with the reduced formelectron mediator. Oxidation reaction of such easy-to-oxidize substancemay sometimes affect the measurement result.

In addition, in the measurement using the sensor as mentioned above, areaction producing hydrogen peroxide using dissolved oxygen as anelectron mediator may proceed concurrently with the reduction of thecarried electron mediator on the reaction layer. Furthermore, thehydrogen peroxide produced by the reaction reoxidizes the reduced formelectron mediator. This may eventually produce a negative error in themeasurement result due to the dissolved oxygen when the substrateconcentration is to be measured based on the oxidation current of thereduced form electron mediator.

The above-mentioned method often applies a voltage between the workingelectrode and a counter electrode to detect liquid junction, namely, todetect supply of sample solution on the basis of an electrical changebetween the two electrodes prior to application of a voltage between theworking electrode and the counter electrode in order to obtain a currentresponse. At that time, it sometimes occurs that measurement startsbefore supply of sufficient amounts of sample solution to the electrodesystem due to a change in resistance value between the above-mentionedworking electrode and the counter electrode, which may sometimes affectthe measurement result. Induction of a change in the condition of aninterface of the working electrode may also affect the measurementresult.

Furthermore, a measurement method with a two-electrode system uses acounter electrode as a reference electrode. This causes a change inpotential of the counter electrode as the standard in association withthe oxidation-reduction reaction at the working electrode, which alsoaffects the measurement result.

The object of the present invention is to eliminate inconveniences asdescribed above and provide a method of determination facilitatingaccurate measurement of substrate concentration by removing influencesof easy-to-oxidize substances.

Another object of the present invention is to provide a method ofsubstrate determination with lesser variations in sensor response.

DISCLOSURE OF INVENTION

The present invention is a method for determining the concentration of asubstrate in a sample solution using a biosensor comprising anelectrically insulating base plate, an electrode system having a workingelectrode, a counter electrode and a third electrode to be used as aninterfering substance detecting electrode, each being formed on theabove-mentioned base plate, and a reaction layer which contains at leastan oxidoreductase and an electron mediator and is formed on theelectrode system omitting the third electrode, wherein the electronmediator is reduced by the generating electrons upon reaction betweenthe substrate contained in the sample solution and the oxidoreductase tomeasure a reduced amount of the electron mediator electrochemically,

the method being characterized by comprising the following steps:

(a) a step of applying a voltage between the counter electrode and thethird electrode;

(b) a step of supplying the sample solution to the reaction layer;

(c) a step of detecting an electrical change between the counterelectrode and the third electrode due to supply of the sample solutionto the reaction layer;

(d) a step of measuring a current flowing between the counter electrodeand the third electrode after the above-mentioned detecting step (c);

(e) a step of releasing voltage application between the counterelectrode and the third electrode after the above-mentioned measuringstep (d);

(f) a step of applying a voltage between the working electrode and thecounter electrode; and

(g) a step of subsequently measuring a current flowing between thecounter electrode and the working electrode.

The present invention also provides a method for determining theconcentration of a substrate in a sample solution using a biosensorcomprising an electrically insulating base plate, an electrode systemhaving a working electrode, a counter electrode and a third electrode tobe used as an interfering substance detecting electrode, each beingformed on the above-mentioned base plate, a reaction layer whichcontains at least an oxidoreductase and an electron mediator and isformed on the electrode system omitting the third electrode, and a covermember forming a sample solution supply pathway to introduce a samplesolution from a sample solution supply port into the above-mentionedreaction layer on the above-mentioned base plate, the third electrodebeing located upstream of the sample solution supply pathway from thereaction layer, wherein the electron mediator is reduced by the producedelectrons upon reaction between the substrate contained in the samplesolution and the oxidoreductase to measure a reduced amount of theelectron mediator electrochemically,

the method comprising the following steps:

(a) a step of applying a voltage between the counter electrode and thethird electrode;

(b) a step of supplying the sample solution to the reaction layer;

(c) a step of detecting an electrical change between the counterelectrode and the third electrode due to supply of the sample solutionto the reaction layer;

(d) a step of measuring a current flowing between the counter electrodeand the third electrode after the above-mentioned detecting step (c);

(e) a step of releasing voltage application between the counterelectrode and the third electrode after the above-mentioned measuringstep (d);

(f) a step of applying a voltage between the working electrode and thecounter electrode; and

(g) a step of subsequently measuring a current flowing between thecounter electrode and the working electrode.

For the method of determination in accordance with the presentinvention, the use of the third electrode as reference electrode ispreferred. Namely, a voltage is also applied between the workingelectrode and the third electrode during the above-mentioned step (f).

When a biosensor with the cover member being integrally combined withthe above-mentioned base plate is used, it is also preferable to providea lecithin carrying layer on an exposed wall surface of the cover memberto the sample solution supply pathway.

It is preferred that the above-mentioned reaction layer further containsa hydrophilic polymer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view illustrating a glucose sensor in accordance withone example of the present invention from which the reaction layer hasbeen omitted.

FIG. 2 is an exploded perspective view illustrating a glucose sensor inaccordance with another example of the present invention from which thereaction layer has been omitted.

BEST MODE FOR CARRYING OUT THE INVENTION

A structure of the biosensor to be used in the method of determinationin accordance with the present invention will be described.

First, a first type biosensor will be described by way of FIG. 1.

In this sensor, a counter electrode 6, a working electrode 7 and a thirdelectrode 8 are formed on an insulating base plate 1 made ofpolyethylene terephthalate, together with respective leads 2, 3 and 4being electrically connected to them. A carbon layer 9 which is formedto facilitate production of reaction layer does not function as anelectrode. A round reaction layer (not shown) containing anoxidoreductase and an electron mediator is formed on the counterelectrode 6, the working electrode 7 and the carbon layer 9 omitting thethird electrode 8. In the figure, numeral 5 represents an insulatinglayer.

Next, a second type biosensor will be described by way of FIG. 2.

This sensor is a combination of the base plate 1 in FIG. 1 with a covermember comprising a cover 10 and a spacer 11. They are bonded to eachother in a positional relationship as shown by the dotted chain line inFIG. 2 to form a sensor.

A slit 12 for forming the sample solution supply pathway is formed onthe spacer 11, and an air vent 13 is formed on the cover 10. Laminatingthe cover 11 on the base plate 1 via the spacer 11 to bond them to eachother results in formation of a cavity which serves as the samplesolution supply pathway at the slit 12 on the spacer 11 by the baseplate 1, spacer 11 and the cover 10. An end edge of this cavitycommunicates with the air vent 13.

In this biosensor, the working electrode 7 is located at a positioncloser to a sample solution supply port 12 a (corresponding to an openedge of the slit 12) than the semilunar counter electrode 6, and thethird electrode 8 is located at a position still closer to the samplesolution supply port 12 a than the working electrode 7. Each of theseelectrodes 6, 7 and 8 is exposed to the above-mentioned cavity.

In measuring the substrate concentration using the above-mentionedbiosensor, an end portion of the sensor to which the leads 2, 3 and 4are provided is set on a measurement device first, followed byapplication of a predetermined potential onto the third electrode 8 withreference to the counter electrode 6. With the potential being applied,a sample solution containing, for instance, ascorbic acid as aninterfering substance is dropped on the reaction layer to dissolve thereaction layer in the sample solution.

Upon supply of the sample solution, a liquid supply detecting systemstarts to operate based on an electrical change between the counterelectrode 6 and the third electrode 8 of the electrode system, which inturn starts a measurement timer. At that time, the potential is keptapplied between the counter electrode 6 and the third electrode 8, and acurrent value between the counter electrode 6 and the third electrode 8is measured when a certain time has passed after detection of supply ofthe sample solution. Since the reaction layer is omitted from the thirdelectrode 8, it takes slight time until the reduced form electronmediator resulting from enzyme reaction reaches near the third electrode8. Therefore, the above-mentioned current value must be derived from theoxidation reaction of the ascorbic acid contained as an interferingsubstance.

Next, the voltage application between the counter electrode 6 and thethird electrode 8 is released.

Subsequently, a potential for oxidizing the above-mentioned reduced formelectron mediator is applied onto the working electrode 7 with referenceto the counter electrode 6 to measure a current value between thecounter electrode 6 and the working electrode 7. This current is derivedfrom the oxidation reactions of the reduced form electron mediator andthe preexisting interfering substance ascorbic acid. In other words, theascorbic acid produces a positive error in the measurement result. Theabove-mentioned current value between the counter electrode 6 and thethird electrode 8 mainly reflects only the concentration of ascorbicacid, so that correction of the measurement result on the basis of thiscurrent value removes any influence of ascorbic acid, whereby an exactsubstrate concentration can be determined.

The second type sensor detects supply of the sample solution between thecounter electrode 6 and the third electrode 8, so that the entireexposed area of the working electrode 7 is filled with the samplesolution with security. As a result, supply of the sample solution canbe determined more reliably.

In the following, the present invention will be described morespecifically by way of examples.

EXAMPLE 1

A method of glucose determination will be described. The base plateshown in FIG. 1 was used for the base plate of a glucose sensor. Theglucose sensor was produced as follows.

A silver paste was printed on the insulating base plate 1 made ofpolyethylene terephthalate by using screen printing to form therespective leads 2, 3 and 4. Next, a conductive carbon paste containinga resin binder was further printed on the base plate 1 to from thecounter electrode 6, the working electrode 7, the third electrode 8 andthe carbon layer 9. The counter electrode 6, the working electrode 7 andthe third electrode 8 are electrically connected to the leads 2, 3 and4, respectively.

Then, an insulating paste was printed on the base plate 1 to form theinsulating layer 5. The insulating layer 5 covers a periphery of each ofthe counter electrode 6, the working electrode 7, the third electrode 8and the carbon layer 9, whereby an exposed area of each of the counterelectrode 6, the working electrode 7, the third electrode 8 and thecarbon layer 9 is held constant. The insulating layer 5 partially coversthe leads 2, 3 and 4.

Next, an aqueous solution of carboxymethyl cellulose (hereinafterabbreviated to CMC) was dropped on the counter electrode 6, workingelectrode 7 and carbon layer 9 omitting the third electrode 8 and driedto form a CMC layer. Dropping an aqueous solution containing GOD as anenzyme and potassium ferricyanide as an electron mediator on the CMClayer once dissolves the CMC layer composed of a hydrophilic polymer,which is formed into a reaction layer by the subsequent drying process,with CMC being mixed with the enzyme and the other constituent. However,the absence of agitation, etc. results in incomplete mixing of the both,whereby the surface of the electrode system is covered with only CMC. Inother words, because of no contact of the enzyme and the electronmediator with the surface of the electrode system, adsorption of proteinonto the surface of the electrode system can be prevented.

In order to measure glucose concentrations using this sensor, an endportion of the sensor at which the leads 2, 3 and 4 are provided was setto a measurement device and a potential of 500 mV was applied onto thethird electrode 8 with reference to the counter electrode 6. With thepotential being kept applied, an aqueous glucose solution containingascorbic acid as an interfering substance was dropped on the reactionlayer as a sample solution at 30 μl. The reaction layer on the electrodesystem dissolved in the dropped sample solution.

Upon supply of the sample solution, a liquid supply detecting systemstarted to operate based on an electrical change between the counterelectrode 6 and the third electrode 8 of the electrode system. Thisstarted a measurement timer. At that time, the potential application isbeing continued between the counter electrode 6 and the third electrode8, and after a lapse of a certain time from the detection of supply ofthe sample solution, a current between the counter electrode 6 and thethird electrode 8 was measured. The current was derived from theoxidation reaction of the ascorbic acid contained as an interferingsubstance and had a proportional relationship with its concentration.After measurement of the current between the counter electrode 6 and thethird electrode 8, the voltage application between the both electrodeswas released.

As mentioned above, the reaction layer was not disposed on the thirdelectrode 8. Therefore, it takes slight time until arrival offerrocyanide ions resulting from enzyme reaction near the thirdelectrode 8. Namely, the current value between the counter electrode 6and the third electrode 8 during an interval until arrival offerrocyanide ions mainly reflects only the concentration of ascorbicacid.

Furthermore, 25 seconds after detection of sample solution, 500 mV wasapplied onto the working electrode 7 with reference to the counterelectrode 6 and a current value between the counter electrode 6 and theworking electrode 7 was measured after 5 seconds.

Reaction of ferricyanide ions, glucose and GOD in the solutioneventually oxidizes glucose to gluconolactone and reduces ferricyanideions to ferrocyanide ions. The concentration of ferrocyanide ion isproportional to the glucose concentration. A current between the counterelectrode 6 and the working electrode 7 after 30 seconds of detection ofthe sample solution is derived from the oxidation reactions offerrocyanide ions and preexisting ascorbic acid. This means thatascorbic acid produces a positive error in the measurement result.However, as described before, the current value between the counterelectrode 6 and the third electrode 8 mainly reflects only theconcentration of ascorbic acid. Therefore, correction of the measurementresult based on that result can remove any effects of ascorbic acidthereby enabling determination of accurate glucose concentration.

EXAMPLE 2

The electrodes 6, 7, 8 and carbon layer 9 were formed on the base plate1 in the same manner as in Example 1. Next, an aqueous CMC solution wasdropped on the counter electrode 6, working electrode 7 and carbon layer9 while omitting the third electrode 8 and dried to form the CMC layer,on which an aqueous solution containing GOD as an enzyme and potassiumferricyanide as an electron mediator was further dropped and dried toform the reaction layer.

Next, for further smoothing supply of the sample solution to thereaction layer, an organic solvent solution of lecithin, such as toluenesolution, for example, was spread from the sample solution supply porttoward the reaction layer and dried to form a lecithin layer on thereaction layer. Next, the cover 10 and the spacer 11 were bonded to thebase plate 1 in a positional relationship as shown by the dotted chainline in FIG. 2 to form a glucose sensor.

The sensor was set on a measurement device and a potential of 500 mV wasapplied onto the third electrode 8 with reference to the counterelectrode 6. With the potential kept applied, an aqueous glucosesolution containing ascorbic acid as an interfering substance wassupplied through the sample solution supply port 12 a at 3 μl as asample solution. The sample solution reached the air vent 13 by passingthrough the sample solution supply pathway and dissolved the reactionlayer on the electrode system.

Upon supply of the sample solution, a liquid supply detecting systemstarted to operate based on an electrical change between the counterelectrode 6 and the third electrode 8 of the electrode system. Thisstarted a measurement timer. At that time, the potential application isbeing continued between the counter electrode 6 and the third electrode8, and after a lapse of a certain time from the detection of supply ofthe sample solution, a current between the counter electrode 6 and thethird electrode 8 was measured. The current was derived from theoxidation reaction of the ascorbic acid contained as an interferingsubstance and had a proportional relationship with its concentration.After measurement of the current between the counter electrode 6 and thethird electrode 8, the voltage application between the two electrodeswas released.

As mentioned above, the reaction layer was not disposed on the thirdelectrode 8. Therefore, it takes slight time until arrival offerrocyanide ions resulting from enzyme reaction near the thirdelectrode 8. Namely, the current value between the counter electrode 6and the third electrode 8 during an interval until arrival offerrocyanide ion mainly reflects only the concentration of ascorbicacid.

Furthermore, 25 seconds after detection of the sample solution, 500 mVwas applied onto the working electrode 7 with reference to the counterelectrode 6 and a current value between the counter electrode 6 and theworking electrode 7 was measured after 5 seconds.

Reaction of ferricyanide ions, glucose and GOD in the solutioneventually oxidizes glucose to gluconolactone and reduces ferricyanideions to ferrocyanide ions. The concentration of ferrocyanide ion isproportional to the glucose concentration. A current between the counterelectrode 6 and the working electrode 7 after 30 seconds of detection ofthe sample solution is derived from the oxidation reactions offerrocyanide ions and preexisting ascorbic acid. This means thatascorbic acid produces a positive error in the measurement result.However, as described before, the current value between the counterelectrode 6 and the third electrode 8 mainly reflects only theconcentration of ascorbic acid. Therefore, correction of the measurementresult based on that result can remove any effects of ascorbic acidthereby enabling determination of accurate glucose concentration.

In the present example, due to detection of supply of the samplesolution between the counter electrode 6 and the third electrode 8, theentire exposed portion of the working electrode 7 is filled with thesample solution with security. This enables still more reliabledetermination of supply of the sample solution.

EXAMPLE 3

A glucose sensor was produced in the same manner as in Example 2.

The sensor was set on a measurement device and a potential of 500 mV wasapplied onto the third electrode 8 with reference to the counterelectrode 6. With the potential kept applied, an aqueous glucosesolution containing ascorbic acid as an interfering substance wassupplied through the sample solution supply port 12 a at 3 μl as asample solution. The sample solution reached the air vent 13 by passingthrough the sample solution supply pathway and dissolved the reactionlayer on the electrode system.

Upon supply of the sample solution, a liquid supply detecting systemstarted to operate based on an electrical change between the counterelectrode 6 and the third electrode 8 of the electrode system, whichthen started a measurement timer. At that time, the potentialapplication is being continued between the counter electrode 6 and thethird electrode 8, and after a lapse of a certain time from thedetection of supply of the sample solution, a current between thecounter electrode 6 and the third electrode 8 was measured. The currentwas derived from the oxidation reaction of the ascorbic acid containedas an interfering substance and had a proportional relationship with itsconcentration. After measurement of the current between the counterelectrode 6 and the third electrode 8, the voltage application betweenthe two electrodes was released.

As mentioned above, the reaction layer was not disposed on the thirdelectrode 8. Therefore, it takes slight time until arrival offerrocyanide ions resulting from enzyme reaction near the thirdelectrode 8. Namely, the current value between the counter electrode 6and the third electrode 8 during an interval until arrival offerrocyanide ions mainly reflects only the concentration of ascorbicacid.

Furthermore, 25 seconds after detection of the sample solution, 500 mVwas applied onto the working electrode 7 with reference to the thirdelectrode 8 and a current value between the counter electrode 6 and theworking electrode 7 was measured after 5 seconds.

Reaction of ferricyanide ions, glucose and GOD in the solutioneventually oxidizes glucose to gluconolactone and reduces ferricyanideions to ferrocyanide ions. The concentration of ferrocyanide ion isproportional to the glucose concentration. A current between the counterelectrode 6 and the working electrode 7 after 30 seconds of detection ofthe sample solution is derived from the oxidation reactions offerrocyanide ions and preexisting ascorbic acid. This means thatascorbic acid produces a positive error in the measurement result.However, as described before, the current value between the counterelectrode 6 and the third electrode 8 mainly reflects only theconcentration of ascorbic acid. Therefore, correction of the measurementresult based on that result can remove any effects of ascorbic acidthereby enabling determination of accurate glucose concentration.

Additional measurement of a potential of the third electrode 8 duringpotential application onto the working electrode 7 with reference to asilver/silver chloride electrode demonstrated almost no change inpotential of the third electrode 8 although oxidation reaction occurredat the working electrode 7. Variations in sensor response were alsodecreased compared to the conventional method which detects liquidjunction based on a change in resistance between the working electrodeand the counter electrode.

EXAMPLE 4

In the same manner as in Example 2, the reaction layer was formed on thecounter electrode 6, working electrode 7 and carbon layer 9 whileomitting the third electrode 8.

Next, an organic solvent solution of lecithin such as toluene solution,for example, was spread on a groove formed on the cover member forforming the sample solution supply pathway and dried, thereby to formthe lecithin layer for the purpose of still more smoothing supply of thesample solution to the reaction layer. Then, the cover 10 and the spacer11 were bonded to the base plate 1 in a positional relationship as shownby the dotted chain line in FIG. 2, which gave a glucose sensor.

Positioning the lecithin layer from the reaction layer over the thirdelectrode 8 may sometimes increase variations in sensor responsecharacteristics due to a change of the surface of the third electrode bythe lecithin layer. Positioning the lecithin layer on the cover memberside as shown above resulted in a decrease in such variations, and theresponse characteristics improved.

EXAMPLE 5

A glucose sensor was produced completely in the same manner as inExample 2 except for omission of the CMC layer from the reaction layer.

And, the result of measurement in the same manner as in Example 2 showeddependency on the ascorbic acid and glucose concentrations despiteincreased variations in sensor response as compared to the case ofincluding the CMC layer.

EXAMPLE 6

A glucose sensor was produced in the same manner as in Example 4.

The sensor was set on a measurement device and a potential of −1,300 mVwas applied onto the third electrode 8 with reference to the counterelectrode 6. With the potential kept applied, an air saturated aqueousglucose solution was supplied through the sample solution supply port 12a at 3 μl as a sample solution. The sample solution reached the air vent13 by passing through the sample solution supply pathway and dissolvedthe reaction layer on the electrode system.

Upon supply of the sample solution, a liquid supply detecting systemstarted to operate based on an electrical change between the counterelectrode 6 and the third electrode 8 of the electrode system, whichstarted a measurement timer. At that time, the potential application isbeing continued between the counter electrode 6 and the third electrode8, and after a lapse of a certain time from the detection of supply ofthe sample solution, a current between the counter electrode 6 and thethird electrode 8 was measured. The current was derived from thereduction reaction of the dissolved oxygen. When a glucose solutiondegassed with argon was supplied, the reduction current decreaseddrastically. After measurement of the current between the counterelectrode 6 and the third electrode 8, the voltage application betweenthe two electrodes was released.

As mentioned above, the reaction layer was not disposed on the thirdelectrode 8. Therefore, it takes slight time until arrival offerricyanide ions in the reaction layer near the third electrode 8.Namely, the current value between the counter electrode 6 and the thirdelectrode 8 during an interval until arrival of ferricyanide ions mainlyreflects only the concentration of dissolved oxygen.

Furthermore, 25 seconds after detection of the sample solution, 500 mVwas applied onto the working electrode 7 with reference to the thirdelectrode 8 and a current value between the counter electrode 6 and theworking electrode 7 was measured after 5 seconds.

Reaction of ferricyanide ions, glucose and GOD in the solutioneventually oxidizes glucose to gluconolactone, and reduction offerricyanide ions to ferrocyanide ions occurs with this oxidationreaction.

On the other hand, a reaction proceeds at the same time as a competitivereaction where dissolved oxygen is reduced to hydrogen peroxide as theglucose is oxidized to gluconolactone due to the action of the dissolvedoxygen in the sample solution as an electron mediator. Hydrogen peroxidegenerating by this reaction reoxidizes ferrocyanide ions to ferricyanideions. Therefore, if glucose concentration is to be measured based on anoxidation current of ferrocyanide ion, such dissolved oxygen can producea negative error in the measurement result.

However, as mentioned before, the current value between the counterelectrode 6 and the third electrode 8 mainly reflects only theconcentration of dissolved oxygen. Therefore, correction of themeasurement result based on that result can remove any effects ofdissolved oxygen thereby enabling determination of accurate glucoseconcentration.

EXAMPLE 7

A glucose sensor was produced in the same manner as in Example 4.

The sensor was set on a measurement device and a potential of 500 mV wasapplied onto the third electrode 8 with reference to the counterelectrode 6. With the potential kept applied, an aqueous glucosesolution containing ascorbic acid as an interfering substance wassupplied through the sample solution supply port 12 a at 3 μl as asample solution. The sample solution reached the air vent 13 by passingthrough the sample solution supply pathway and dissolved the reactionlayer on the electrode system.

Upon supply of the sample solution, a liquid supply detecting systemstarted to operate based on an electrical change between the counterelectrode 6 and the third electrode 8 of the electrode system, whichstarted a measurement timer. At that time, the potential application isbeing continued between the counter electrode 6 and the third electrode8. Two seconds after detection of supply of the sample solution, thepotential to be applied onto the third electrode 8 was changed to −1,300mV. The current between the counter electrode 6 and the third electrode8 was measured at two time points immediately before and 3 seconds afterthe potential change to −1,300 mV. The current immediately before thepotential change is mainly dependent on the concentration of ascorbicacid. On the other hand, the current 3 seconds after the potentialchange to −1,300 mV is mainly dependent on the concentration ofdissolved oxygen in the sample solution.

After measurements of the current between the counter electrode 6 andthe third electrode 8 after 2 and 5 seconds of supply of the samplesolution, the voltage application between the two electrodes wasreleased.

Twenty-five seconds after detection of the sample solution, 500 mV wasfurther applied onto the working electrode 7 with reference to the thirdelectrode 8 and the current between the counter electrode 6 and theworking electrode 7 was measured after 5 seconds.

As described above, the current value between the counter electrode 6and the third electrode 8 mainly reflects concentrations of ascorbicacid and dissolved oxygen. Therefore, concentrations of those twosubstances can be determined based on that current value. Therefore,correction of the measurement result based on that result can remove anyeffects of ascorbic acid and dissolved oxygen thereby enablingdetermination of accurate glucose concentration.

In the foregoing examples, although the potential to be applied onto thethird electrode 8 for sensing supply of the sample solution to detectascorbic acid or dissolved oxygen was 500 mV or −1,300 mV, the presentinvention is not limited to those potential values. Moreover, although apotential of 500 mV was applied onto the working electrode 7 to obtain aresponse current, the present invention is not limited to this potentialvalue and any potential may be used if it can oxidize the reduced formelectron mediator resulting from a series of reaction. The time point tomeasure the current value is also not limited to those used in theforegoing examples.

In the foregoing examples, although carboxymethyl cellulose was used asthe hydrophilic polymer, a variety of hydrophilic polymers can be usedfor forming the hydrophilic polymer layer. Exemplary hydrophilicpolymers include hydroxyethyl cellulose, hydroxypropyl cellulose, methylcellulose, ethyl cellulose, ethylhydroxyethyl cellulose,carboxymethylethyl cellulose, polyvinyl pyrrolidone, polyvinyl alcohol,polyamino acid such as polylysin, polystyrene sulfonate, gelatin and itsderivatives, polyacrylic acid and its salts, polymethacrylic acid andits salts, starch and its derivatives, and a polymer of maleic anhydrideor a maleate. Of them, carboxymethyl cellulose, hydroxyethyl celluloseand hydroxypropyl cellulose are preferred.

The oxidoreductase to be contained in the reaction layer is selecteddepending on the substrate contained in the sample solution. Exemplaryoxidoreductases include fructose dehydrogenase, glucose oxidase, alcoholoxidase, lactate oxidase, cholesterol oxidase, xanthine oxidase, andamino-acid oxidase.

As the electron mediator, potassium ferricyanide, p-benzoquinone,phenazine methosulfate, methylene blue, and ferrocene derivatives may beexemplified. These electron mediators may be used singly or incombination of two or more.

The above-exemplified enzymes and electron mediators may be dissolved inthe sample solution or may be prevented from dissolving in the samplesolution by fixing the reaction layer onto the base plate and so on.When the enzyme and the electron mediator are to be fixed, the reactionlayer preferably contains the hydrophilic polymer.

In the foregoing examples, specific electrode systems were shown, butthe present invention is not limited to those electrode systems withrespect to the shape of the electrode and location of the electrodes andleads.

In the foregoing examples, although carbon was used as the material ofthe third electrode, the present invention is not limited to carbonelectrode and those made of other conductive material or a silver/silverchloride electrode can also be used.

INDUSTRIAL APPLICABILITY

As discussed above, the present invention enables substratedetermination of high reliability.

What is claimed is:
 1. A biosensor for determining the concentration ofa substrate in a sample solution, said biosensor comprising: anelectrically insulating base plate; an electrode system having a workingelectrode, a counter electrode and a third electrode to be utilized asan interfering substance detecting electrode, said working electrode,said counter electrode and said third electrode being formed on saidbase plate; a reaction layer comprising an oxidoreductase and anelectron mediator, said reaction layer being formed on said electrodesystem so as to cover said working electrode and said counter electrode,said reaction layer not covering said third electrode; and a covermember forming a sample solution supply pathway to introduce a samplesolution from a sample solution supply port into said reaction layer onsaid base plate, wherein said third electrode being located closer tosaid sample solution supply port than said reaction layer and saidentire working electrode being located closer to said sample solutionsupply port than said counter electrode.
 2. The biosensor in accordancewith claim 1, herein said working electrode is located closer to saidsample solution supply port than said counter electrode so that theentire exposed area of said working electrode can be filled with asample solution prior to reaching said counter electrode.
 3. Thebiosensor in accordance with claim 1, wherein said working electrode,counter electrode and third electrode are electrically connected tothree independent leads.
 4. The biosensor in accordance with claim 1,wherein said biosensor is disposed with a layer essentially composed oflecithin on an exposed surface of the sample solution supply pathway ofsaid cover member.
 5. The biosensor in accordance with claim 1, whereinsaid biosensor further contains a hydrophilic polymer in said reactionlayer.